Penicillin and more recently cephalosporins have gained substantial prominence as medicaments for the treatment of microbial infections. Common to the structure of these two classes of bicyclic antibiotic compounds is a .beta.-lactam ring system. Until the recent discovery of nocardicin, a monocyclic azetidinone compound having the formula ##STR1## researchers in the area of .beta.-lactam antibiotics have generally believed that a bicyclic .beta.-lactam containing ring structure is essential to achieve the high levels of antibiotic activity characteristic of the clinically significant penicillins and cephalosporins. Although many monocyclic .beta.-lactam compounds have been known, interest in these compounds has been primarily in their use a intermediates to penicillins, cephalosporins and other bicyclic compounds. The recent discovery of nocardicin has prompted investigators to take a closer at monocyclic .beta.-lactam compounds and their antimicrobial activity. The present invention is directed to novel 2-unsubstituted-3-acylaminoazetidin-4-one compounds which exhibit antibiotic activity.